The advent of gravitational wave detectors – there are now four of them – has recorded a steady stream of black hole mergers. As far as we can tell, nearly all of them behaved exactly as we expected the kinds of events we predicted would produce them: a pair of orbiting black holes that gradually spiral inward until until they meet at their mutual center of gravity. .
But there was one event that apparently didn’t quite match the kinds of signals we were expecting. And the researchers now suggest it was the product of something that should be incredibly rare: two black holes found in the vastness of space. After a single close pass, the two bodies hunched over and immediately tipped over in a collision.
Models and Tweets
Black hole collisions require the two black holes to be close enough to each other to interact gravitationally. Since space is so vast, that would usually mean that they are the products of two massive stars that formed as a binary system. After the stars died and left black holes behind, the two bodies would slowly spin towards each other, radiating energy in the form of gravitational waves as they do.
This leads to relatively simple inspiration and fusion, the details of which have appeared in countless animations following LIGO’s first detection of a black hole collision.
Collisions of this type are so well worked out that we have a large number of simulations that model a collision like this with different sets of details: different black hole masses, different spins, etc. These simulations provide “models” of the final moments before collisions, when gravitational wave production becomes both faster and more intense, with the final “chirp” of waves rising above the background noise on Earth. These patterns allow us to quickly identify the details of a crash, based on how closely the crash signals match one of these patterns.
But a merger called GW190521 didn’t fit the models really well, and only matched better if the black holes involved weren’t spinning at all. The tweet was unusually short and there is no sign of a signal before the actual meltdown. Finally, the two objects involved in the merger were relatively massive: around 50 and 80 times the mass of the Sun. Black holes of this size do not form in supernovae (these usually start with less than 15 solar masses), so they are likely the products of past collisions. Which makes starting them as part of a binary system a questionable proposition.
Thus, a team of European researchers decided to model an event that should be relatively rare: the two black holes did not start in a mutual orbit, but happened close enough to gravitationally cling to each other. other.
Shall we dance?
The technical term for what the authors propose is “dynamic capture”, which explains the seemingly sudden and burst nature of the GW190521 signal. Instead of the gradual approach where gravitational waves increase in intensity that characterizes binary systems, the two bodies that triggered this event could experience a limited number of high-speed oscillations past each other before colliding.
The researchers modeled a variety of potential approaches, some of which would lead to a gradual approach similar to that seen in binary systems and others that could push the two black holes away from each other on altered trajectories. But between the two extremes is a set of results where you could either have a small number of close passes before colliding, or the two black holes could plunge directly into each other.
The models that produced a chirp that best matched the GW190521 signal saw a single pass that brought the black holes closer together, followed by a single fast curve in the collision. But the first pass was far enough away that the signal was too weak to stand out from the background noise in the detectors. Although it is possible to produce results similar to these using a more typical collision profile with gradual inspiration, various statistical tests suggest that dynamic capture is more likely.
It’s probably based on the properties of gravitational wave chirping, at least. The likelihood of two black holes getting close enough to each other to trigger the process is another matter altogether. But both of these black holes are massive enough to have been built up by previous mergers, suggesting that this collision took place in a dense cluster where many massive stars are dying. Thus, the environment may be more conducive to a chance encounter than one might expect.
natural astronomy2022. DOI: 10.1038/s41550-022-01813-w (About DOIs).
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